This invention relates to processing a video signal from a source, to convert from a lower dynamic range such as conventional, standard dynamic range (SDR), to a signal usable by devices having a higher dynamic range.
High Dynamic Range video (HDR) is a new technique which allows the content producer to more accurately reproduce an image without the suppression of highlights usually associated with conventional video. HDR video is currently being debated across the industry, with methods to capture, process, transmit and display HDR video being discussed in standards bodies, trade associations, national regulators and broadcasters globally. Further, manufacturers in the consumer display industry have started to create screens with HDR performance marketed under proprietary names such as eXtended Dynamic Range or Super UHDTV and professional capture devices are being joined by prosumer and handheld capture devices aimed at the early adopter market.
HDR video has a dynamic range, i.e. the ratio between the brightest and darkest parts of the image, of 10000:1 or more. Dynamic range is sometimes expressed as “stops” which is logarithm to the base 2 of the dynamic range. A dynamic range of 10000:1 therefore equates to 13.29 stops. The best modern cameras can capture a dynamic range of about 14 stops and this is improving as technology develops. Conventional televisions (and computer displays) have a restricted dynamic range of about 100:1. This is sometimes referred to as standard dynamic range (SDR).
However, large broadcasters have large archives of SDR video and photographic material, in many cases running into the many millions of hours, which will continue to be used in programmes created for an HDR channel. For example, an historical documentary will continue to use archive footage. For events coverage, it may take many years for a full technical refresh of cameras to occur, so broadcasters may be required to use a mixture of HDR and SDR camera equipment. Similarly, consumers will have many hours of purchased and personal videos stored on tape, optical disk and file server which they will want to continue to watch.
Our earlier patent application UK 2526047 provides an approach to producing a signal viewable on both HDR and non-HDR displays by providing a non-HDR video signal within a Hybrid Log-Gamma (HLG) HDR video signal. This method assumes that the video image should be displayed as it would be on a top-of-the-range non-HDR consumer display and simply positions the non-HDR signal within the HDR container. HLG is summarized in ITU report ITU-R BT.2390-0
At present SDR video may be converted to HDR video through the process of “colour grading” or simply “grading”. This is a well-known process, of long heritage, in which the colour and tonality of the image is adjusted to create a consistent and pleasing look. Essentially this is a manual adjustment of the look of the video, similar in principle to using domestic photo processing software to change the look of still photographs. Professional commercial software packages are available to support colour grading. However the process of colour grading requires the use of a skilled operator, is time consuming and, therefore expensive. Furthermore it cannot be used on “live” broadcasts such as sports events.